[hardwired]

You had the constructor code inside the main method and the actionPerformed method, required for the enclosing class implementing the ActionListener interface, was crossed up with the constructor signature: public class PatientDetails (ActionEvent addingInfo).

Code:

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.io.*;

public class PD extends JFrame implements ActionListener
{
    // This class extends JFrame so it is a JFrame
    // and you don't need to create a new instance.
//    JFrame frame;
    JPanel panel;
    JLabel patientFirstName;
    JLabel patientMiddleName;
    JLabel patientLastName;
    JLabel patientPhoneNumber;
    JLabel patientPostalAddress;
    JTextField textPatientFirstName;
    JTextField textPatientMiddleName;
    JTextField textPatientLastName;
    JTextField textPatientPhoneNumber;
    JTextField textPatientPostalAddress;
    JComboBox patientChoice;
    JList paymentOption;
    JButton button;

    public PD()
    {
        super("PatientDetails");
        panel = new JPanel (new GridLayout(0,1));
        getContentPane().add(panel);
//        panel.setLayout (new FlowLayout ());
 
        patientFirstName = new JLabel ("First Name: ");
        textPatientFirstName = new JTextField (15);
 
        patientMiddleName = new JLabel ("Middle Name: ");
        textPatientMiddleName = new JTextField (15);
 
        patientLastName = new JLabel ("Last Name: ");
        textPatientLastName = new JTextField (15);
 
        patientPhoneNumber = new JLabel ("Phone Number: ");
        textPatientPhoneNumber = new JTextField (15);
 
        patientPostalAddress = new JLabel ("Address: ");
        textPatientPostalAddress = new JTextField (50);
 
        String listOfPaymentOptions [] = {"Credit Card", "Cash", "Check"};
        paymentOption = new JList (listOfPaymentOptions);
 
        String memberChoice [] = {"New Patient", "Regular Patient", "Guest Patient"};
        patientChoice = new JComboBox (memberChoice);
 
        button = new JButton ("Submit");
 
        JPanel row1 = new JPanel();
        row1.add(patientFirstName);
        row1.add(textPatientFirstName);
        row1.add(patientMiddleName);
        row1.add(textPatientMiddleName);
        row1.add(patientLastName);
        row1.add(textPatientLastName);
        panel.add(row1);

        JPanel row2 = new JPanel();
        row2.add(patientPhoneNumber);
        row2.add(textPatientPhoneNumber);
        panel.add(row2);

        JPanel row3 = new JPanel();
        row3.add(patientPostalAddress);
        row3.add(textPatientPostalAddress);
        panel.add(row3);

        JPanel row4 = new JPanel();
        row4.add(paymentOption);
        row4.add(patientChoice);
        row4.add(button);
        panel.add(row4);

        // JFrame methods
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        pack();
        setLocation(10,200);
        setVisible(true);
    }

    public void actionPerformed(ActionEvent addingInfo)
    {
        Object abc = addingInfo.getSource ();
        if (abc == button)
        {
            String entry = textPatientFirstName.getText () + ":" +
                           textPatientMiddleName.getText () + ":" +
                           textPatientLastName.getText () + ":" +
                           new String(textPatientPhoneNumber.getText()) + ":" +
                           new String(textPatientPostalAddress.getText());
            try
            {
                RandomAccessFile patientFile =
                    new RandomAccessFile ("C:\\Documents and Settings\\Ryan\\" +
                                          "My Documents\\File.txt", "rw");
                patientFile.seek(patientFile.length());
                patientFile.writeBytes (entry);
            }
            catch (IOException error)
            {
                System.out.println("Cannot write to file" +error);
            }
        }
    }

    public static void main (String args[])
    {
        new PD();
    }
}